class_docstr: Consider implementing simple np sum method of label tags; Also add valid slices that have label mask
code: def __call__(self, request, datastore: Datastore):
      loader = LoadImage(image_only=True)
      result = {}
      for image_id in datastore.list_images():
          for tag in self.tags:
              label_id: str = datastore.get_label_by_image_id(
                  image_id, tag
              )
              if label_id:
                  label = loader(datastore.get_label_uri(label_id, tag))
                  if isinstance(label, torch.Tensor):
                      label = label.numpy()
                  slices = [
                      sid
                      for sid in range(label.shape[0])
                      if np.sum(label[sid] > 0)
                  ]
                  info = {
                      "sum": int(np.sum(label)),
                      "slices": len(slices),
                  }
                  logger.debug(f"{label_id} => {info}")
  
                  datastore.update_label_info(label_id, tag, info)
                  result[label_id] = info
      return result
  
file_path: MONAILabel/monailabel/tasks/scoring/sum.py
imports: import logging
  import numpy
  import torch
  from monai.transforms import LoadImage
  from monailabel.interfaces.datastore import Datastore, DefaultLabelTag
  from monailabel.interfaces.tasks.scoring import ScoringMethod
label_desc: Use image node_id 2 for calling the Sum obj's underlying member method code with example usage: obj.__call__(request, datastore) and returns: result